1. Field of the Invention
The present invention relates to a cathode ray tube, and more particularly, to a color selection apparatus having a tension mask that performs a color selection function for the realization of color images in a cathode ray tube.
2. Description of the Related Art
A cathode ray tube (CRT) is typically a display device in which three electron beams emitted from an electron gun scan a phosphor screen to realize predetermined images. A color selection apparatus is provided within a bulb that forms an exterior of the CRT and at a position next to the phosphor screen. The color selection apparatus includes a shadow mask that separates the three electron beams so that the electron beams land correctly on red (R), green (G), and blue (B) phosphor layers of the phosphor screen, and a frame for supporting the shadow mask.
The shadow mask of the color selection apparatus is formed by performing a photolithography process on aluminum-killed steel or INVAR steel of an extremely small thickness to form a plurality of electron beam passage apertures, after which drawing is performed to realize the final desired form. Since the thickness of the shadow mask is extremely small relative to its area, and because of the formation of hundreds of thousands of the apertures, the shadow mask becomes structurally weak. This leads to the shadow mask becoming depressed after receiving an outside shock or thermal expansion (i.e., doming) toward the phosphor screen as a result of the electron beams that do not pass through the apertures striking the shadow mask.
With such deformation of the shadow mask, the positioning of the apertures is altered from a state at which it is initially formed such that precise color selection of the electron beams does not occur. This causes mislanding of the electron beams to thereby result in reduced color purity of the screen.
Therefore, to overcome this problem and also enable manufacture of large screen sizes and flatter profiles, shadow masks have been developed that are fixed to a frame while in a state of tension (hereinafter referred to as a tension mask).
A conventional color selection apparatus for a cathode ray tube includes a tension mask that has formed in an inner area thereof a plurality of electron beam apertures. The tension mask is fixed to support members of a frame in a state where the tension mask receives tension along a direction of its short sides. The tension mask is fixed to mounting portions of the support members by welding the tension mask to the mounting portions. Also, an elastic member extends between the two support members on both sides of the tension mask to thereby complete the structure of the frame.
First, in a state whereby they are connected to the elastic members, a force of a predetermined magnitude is applied to the support members in a direction toward each other so that the support members are positioned closer together. As a result, the elastic members are elastically deformed at bending portions such that the elastic members and the support members provided thereon are positioned as shown by the dotted lines in the drawing.
In this state, the tension mask is fixed by welding to the support members 106, after which the force is removed. Therefore, the elastic members return toward their original position, that is, the elastic members return toward a position that is parallel to a resistive force of the tension mask. Tension is formed in the tension mask as a result.
However, with the bending of both ends of each of the elastic members as described above, permanent depressions are formed in the bending portions. The depressions weaken the bending portions of the elastic members so that an insufficient tensile force is supplied to the tension mask by the elastic members.
Further, after the tension mask is welded to the support members of the frame, there results a significant difference (of approximately 4 mm) between an amount of plastic deformation at ends and at center areas of the support members. This difference in deformation amounts along a length of the support members results in differences in the amount of tension of the tension mask over the same area.
A direct consequence of the tension mask having different levels of tension over its area is that the tension mask exhibits differences in natural frequency depending on location. If the CRT utilizing such a conventional color selection apparatus is mounted in a television, for example, the sound output from the speakers affects the tension mask such that distances between the same and the phosphor screen vary to differing degrees depending on the location of the tension mask. As a result, the electron beams passing through the apertures of the tension mask do not land on their intended positions (i.e., the intended phosphors) so that picture quality deteriorates.
In sum, as a result of the depressions formed in the elastic members of the frame in the conventional color selection apparatus, an insufficient amount of tension is applied to the tension mask such that the mask vibrates to ultimately result in a deteriorated picture quality. This problem is made worse by the fact that the amount of tension applied to the tension mask varies at different locations so that the tension mask vibrates differently at different areas.